Combustion guide for aircraft engines



1941- c. c. MINTER 2,251,751

COMBUSTION GUIDE FOR AIRCRAFT ENGINES Filed Oct. 29, 1937 LENSULA'ILION o 75 i Patented Aug. 5, 1941 Clarke (J. Minter, East Orange, N. 1., assignoi to Breeze Corporations, Incorporated, Newark, N. 1., a corporation of New Jersey Application October 29, 1931, Serial No. 171,802 (01. 13-51) -6 Claims.

This invention relates to a combustion guide for aircraft engines; and it comprises an instrument adapted for attachment to internal combustion engines to guide the pilot in adjustin fuel-air ratios in correlation with throttle variations and wherein a calibrated electrical resistance varying in proportion to combustion temperature is balanced in a Wheatstone bridge'with a calibrated resistance actuated to vary in proportion to the square root of intake manifold pressure and a null point electric meter across the bridge is actuated by unbalanced conditions in the bridge to indicate changes required in the fuel feed for adjustment of the combustion temperature to changes in throttle feed, altitude, etc.;

all as more fully hereinafter set forth and as claimed. 1

In the operation of internal combustion engines with air cooling, as in aircraft engines, one of the outstanding problems is to prevent overheating of the engine parts without too great sacrifice of power and fuel economy. It is known that the temperature reached in the combustion approaches a maximum as the relative proportions of air and fuel in the charge mixture come near to the quantities required for complete combustion of the fuel. It is customary to operate with a rich" mixture, that is, with an excess of fuel over the ratio required for complete combustion with the air giving maximum temperature. The excess of fuel lowers the combustion temperature.

Usually, the fuel feed is adjusted to give a charge mixture rich enough to avoid all danger of overheating. This is often at a serious sacrifice of fuel economy. Aircraft engine builders as a rule recommend fuel-air ratios that give the best economy possible without danger of overheating.

with the thermal conductivity of the exhaust gas and in the other arm a constant resistance associated with the thermal conductivity of air. A millivoltmeter connected across the bridge measures disturbances of the equilibrium of the bridge due to changes in the composition and thermal conductivity of the exhaust gas, thereby reflecting changes in the combustion temperature, associated with changes in mixture ratio.-

A mixture indicator operating on this principle is described in my Patent No. 2,025,121 of Dec. 24,

With apressure gauge on th intake manifold, and a fuel mixture ratio indicator, as described, the engine operator can adjust the fuel feed to the carburetor to avoid overheating of the engine. He may enrich the fuel mixture as the intake pressure is raised upon opening the throttle drive and vice versa.

In the present invention I provide correlated registration of intake pressure and mixture ratio by a single indicating mechanism responding to both of these factors and thus to the product of the quantity of heat and the temperature of combustion. The heating effect of the combustion is measured by the product of the quantity and intensity factors. To obtain correlated joint registration of the two factors, I arrange in a Wheatstone bridge two resistances varied respectiveiy by changes of intake. manifold pressure and of mixture ratio or exhaust temperature with a null point electric meter giving indication of unbalancing of the bridge as effected by change in either or both of the two variable resistances.

Change of the resistance in one arm of the bridge is made automatically responsive to the The mixture ratio can be varied by adjustment of I the carburetor according to the throttle or rate of driving which controls the gas pressure in the intake manifold of the engine delivering the charge mixture of air and fuel to the cylinders.

It has become common practice to include as parts of aircraft engine equipment a pressure gauge indicating intake manifold pressure and a mixture ratio dicator gives an indirect measurement of the mixture ratio delivered by the carburetor, and thus of the combustion temperature, by measuring intake manifold pressure by suitable means and a resistance associated with a certain throttle opening and the corresponding intake pressure is made to balance with a resistance in the other I v arm associated with the predetermined correct indicator. This mixture inthe thermal conductivity of the exhaust gas which varies with its composition and thuswith the ratio of fuel to "air in the charge mixture. The indicator comprises a Wheatstone bridge having in one arm a variable resistance associated mixture ratio and associated combustion temperature for the chosen intake pressure so that the meter pointer rests on the null point indicating that the charge mixture is of the proper richness to avoid overheating. Further opening of the driving throttle increase the intake gas pressure or charge density and this, automatically reflected in change of the electrical resistance of one arm of the bridge, imbalances the bridge and the meter pointer moves of! the null pointto indicate that the mixture is too lean, The

operator is thus told that to avoid overheating the mixture needs anlincreased ratio of fuel to air and the fuel feed at the carburetor can be increased until the pointer returns to the null point of the meter. If the mixtur becomes too root of the heat quantity.

rich, as by an increase of altitude, or of the intake air temperature, the bridge resistances beu come unbalanced in an opposite sense making the meter pointer move off in the opposite direction indicating richness, to be brought back to the null point by a decrease of fuel feed, leaning the mixture. As long as the pointer rests at the null point the operator knows that the mixture ratio is safe for the throttle opening'or driving rate. r

Thus there is a correlated or oint registration brated variable Wheatstone bridge resistances of the two combustion'factors by means of calimade respectively responsive to intake gas pressure or charge density and to mixture ratio or exhaust and combustion temperatures.

The heating efiectof the combustion upon the engine is due to the factor of amount or rate of rate of oxygen feed as reflected in the charge density which I designate the quantity factor and to the factor of combustion temperature-dependent mainly upon the fuel-air ratio in the charge mixture and I designate. the combustion temperature the intensity factor in engine overheating. I

It has been determined experimentally that the rate at which the hot products of combustion lose heat to the engine parts in contact with the gases is proportional to the approximate square root of the charge density or quantity factor in combustion andis also proportional to the temperature diiference'between the hot gases and j the engine surfaces, cylinder walls, pistons, etc.

Upon these observations I have formulated a rule for the safe operation of an air-cooled aircraft engine; a rule requiring that the product of the 1 intensity factor and the square root of the quantity factor should be kept constant.

amount of heat liberated in unit time increases, the temperature of the combustion should be re- As the duced in proportion tothe increase in the square In the present invention I utilize as heat development, proDOItional to the amount or changed in density by the volume change responding to change of manifold pressure.

The temperature or intensity factor is made 'to vary the balance of the bridge resistances most advantageously by means of a resistance thermometer in the exhaust manifold and connected into one arm of the bridge. If desired, however,

the intensity factor resistance may be varied by a conventional thermal conductivity cell arrangement.

In the accompanying drawing are shown in diagrammatic form combustion guide instruments-within my invention. In this showing,

Fig. 1 is a diagrammatic showing of a combustion guide utilizing two opposing Wheatstone bridge resistances varying respectively with the.

square root of intake manifold pressure and with exhaust gas temperature;

Fig. 2 shows in diagram a modification of the combustion guide wherein one resistance is made responsive to intake pressure by mechanical contact means actuated by a sylphon responding to intake pressure and the opposing resistance is varied by a thermal conductivity cell connected to the exhaust manifold;

Fig. 3 is a diagrammatic showing of apparatus,

including one resistance responding directly to the temperature of exhaust gas and the opposing resistance to thermal conductivity of agas varied in volume by intake pressure changes; and

Fig. 4 shows the combination in a Wheatstone bridge of one resistance varying with thermal conductivity of a gas in proportion to the square root of intake pressure opposite to another resist- 1 ance varied by a thermal conductivity cell connected to the exhaust.

Referring to Fig. 1 a closed casing or chamber I0 is connected to the intake manifold I l (leading I to the engine cylinders) by conduit I2. A partially evacuated Sylphon bellows I3 of metal construction is placed in the sealed casinglll. In

' the casing a tapered loop resistance l5 has a a measure of the quantity factor the intake manifold pressure with which the quantity factor varies directly.

For the intensity factor I utilize the closely related composition or temperature of the exhaust gases.

I transform these two factors into variable Wheatstone bridge resistances and get a joint correlated registration of the two factors enabling the engine operator to keep the product of the two factors constant as required by the rule. The null point of the bridge meter corresponds with this constant product.

I may use any suitable means for varying the resistance in one arm of the Wheatstone bridge inproportion to variation of the square root of the intake manifold pressure. For example, this is advantageously effected by an evacuated Sylphon bellows in a chamber having communication with the intake manifold and mechanically linked to an arm moved across a sized resistance wire with electrical connection to the bridge meter. Another advantageous means for effecting pressure responsive resistance variation is a partially evacuated bellows and communicating cell containing a gas such as air or nitrogen under a pressure of less than one inch of mercury for example and containing a wire resistance connected in the bridge and varying with the thermal conductivity of the gas in the bellows system as contact arm l6 moved by the bellows asthe latter is contracted and expanded by increase and decrease of pressure in chamber II) which varies directly with variation of the intake. pressure of air and chargedfuel as communicated through conduit l2 to the chamber. As shown, the con} tact arm i6 is moved upon the loop resistance 15 by a mechanical linkage actuated by the bellows and comprising a linking member l1, pivoted on standard l8, with a pivoted linkage l9 to the bellows at one end and with a slot and pin connection 20 at the other end to a' pivoted rack and pinion 22 which moves the arm l6 around loop I 5. Member I1 is sulated by suitable means from ion 22.

The wire loop resistance I5 is connected in electrical series between'the series branches desthe rack and pinignated B and D of a Wheatstone bridge which' are in parallel to the other two branches in series designated A and C. -The loop l5, as shown in Fig. 1 is connected at one end to branch D of the bridge and to branch 3 through arm. l6 and the rack and pinion 2 2. The rack and pinion also are connected to the bridge electrically inlead 21' in branch B.

T 2,251,751" v variable with-the square root of the intake presering the CO: content of the exhaust, increases sure forms a part of branch D ot'the bridge the thermal conductivity in cell i, lowersthe which is opposite branch A. onnected into temperature and thus the resistance of the spiral branchAbyleadflisthespilalreslstancefl of a resistance thermometer located in the ex-- haustmanifoldoftheengineandvaryingdb,

battery to-the bridge, as shown, is such as to balance against'lead. in branch A a second The resistance of branches BandCmaybeapproximately equal.

-ca'sing01fusdquartz. Theeonnectionfromthe In operation, the bridge being in balance when the product of the A and D resistances is equal and restores the product of the A and D resistances to its equality with the product of the B and C resistances. The bridge is balanced,

and the meter returned to the null point in accor-dance with the rule that the pr duct of the intensity factor and the square root of-the quantity factor should be kept constant.= Pig. -3 shows an alternative modification of means for automatic variation oi the bridge balance with increase or decrease of intake pressure. A sealed chamber 35 incommunication through conduit 38 with the air-fuel intake system. ii encloses a partially evacuated Sylphon bellowsfl which communicates with a cell 38 in which is sealed a spiral wire resistance. The

A and D resistances shall be constant. with the loop resistance l5 calibrated to vary according to the square root of the intake pressure, then as the loop I! resistance is increased with the counter-clockwise movement of am it by the bellows linkage in response to increased intake pressure, the added resistance disturbs the bridge balance, the meter pointer movesto the lean side and the instrument registers automatically the departure from the requirement that the product of the square root of the heat quantity and the temperature intensity shall remain constant. Likewise the instrument registers restoration of theproper combustion conditions when the bridge is returned to balance and the meter is moved back to the null point by an increase of the fuel feed to the carburetor which raises the mixture ratio and reduces the combustion and exhaust temperatures, lowering the A resistance'of the bridge in the proper ratio to keep the product of the A and D bridge resistances constant when the product of the temperature factor and the square root of the pressure factor is constant; the null point of the meter corresponding to this constant product.

In the arrangement shown in Fig. 2, the loop resistance 15 is connected in series with branches B and D through leads 29 and 30. Lead connects the contact arm it through its pinion and rack to the bridge meter. Fig. 2 illustrates use of the relative thermal conductivity of the exhaust gas as a measure of the intensity or temperature factor of combustion. A cell 3! in communication with the exhaust manifold E contains a spiral'wire resistance connected into the bridge as branch A in parallel to branch B in which a resistance wire contained in an air cell 32 with a vent I3 is connected. The fixed resistances in branches C and D may be approxiproduct of the variable resistances of branches A and D is equal to the constant product of the B and C resistances. Thus the A resistance varying with the thermal conductivity ofthe exhaust .gas and thus with its composition and the combustion temperature-is opposed in the vbridge to the resistance varied by changes in intake pressure. When intake pressure rises,

partial evacuation of the bellows and communicatingcell is to a low pressure of the order of 1' inch of mercury; air and nitrogen being suitable gases to be contained in the bellows-cell system. The resistance of the wire in cell 38 under an impressed current varies inversely with the thermal conductivity'of the gas in the system 7 and this in turn depends upon the density oi the gas. Increase of the intake pressure communicated to chamber 35 contracts the volume of bellows 31 and increases the density of the gas in cell 38 and its thermal conductivity. Thus the temperature and resistance of the wire in cell 38 is decreased as the intake pressure rises.

The resistance can be calibrated to vary inversely with the square root of the intake pressure.

As shown in Fig, 3, the cell 38 resistance is connected in a Wheatst'one bridge as branch D in series with a resistance thermometer at in the exhaust clan lntemal combustion engine which forms branch B of the bridge. Branch A contains a fixed resistance and branch C a resistance sealed in a cell 39 containing air and fixed according to the thermal conductivity of.

the air. The resistance in air cell 39 is opposed inthe bridge to the variable thermometer resistance.

In operation, the bridge balance requiring equality of the resistance products AD and BC, the bridge is unbalanced and registers lean when the engine throttle opening is increased and the increased pressure in chamber 35 contracts the gas volume in bellows 31 and cell 38. This,-

by increasing thegas density is reflected in an increase of thermal conductivity and a decreasev of the branch D resistance which creates a. bridge potential throughthe meter M. The balance is .restored when increase of the fuel ratio lowers the thermometerresistance, thus decreasing the BC product to compensate for the decrease of the AD product by the rise in' the intake pressure. The product of the intensity factor and the square root of the quantity factor is kept constant and danger of overheating the engine is avoided.

In the instrument of Fig. 4, the arrangement of the partially evacuated Sylphon bellows and thermal conductivity cell 38 actuated by intake arm It moves clockwise, increasing the branch D resistance in the square root ratio, unbalancing the bridge and making the meter registerpressure is similar to that of Fig. 3. The gas pressure in the bellows and cell, low enough to give a relatively large variation of thermal conductivity withincrease or decrease of pressure,

makes for sensitivity of the cell 38 resistance to intake pressure changes and ready calibration of the resistance to the square root variation. In,

the wheatstone bridge arrangement, however,

pensated by a change of exhaust gas composition in the same direction as that of the B resistance but opposite to that of the intake pressure. The bridge is put out of balance by a change in either the A or the B resistance responding respectively to change in the exhaust gas composition and to change in the square root of the intake pressure.

chambers, said arms being in parallel with one another and in series with the said source of current, said meter being operated by variations in theresistances of the Wheatstone bridge whereby the differential between the intake manifold pressure and the exhaust manifold temperature is indicated on said meter.

4. A combustion guide for internal combustion engines comprising a source of electric current,

a Wheatstone bridge arrangement connected with said source, a meter connected acrossthe Wheat- Thus both the quantity and intensity factors'in combustion are registered in, correlation by the bridge meter and the operator is guided to keep the product of the two factors constant as required for safety from overheating. In all cases, the engine operator is guided inadjustin'gthe combustion conditions to avoid overheating without undue sacrifice of power and of fuel economy.

What I claim is: v r

1. A combustion indicatorforinternal combustion engines comprising a Wheatstone bridge having in one of its branches a variable resistance responsive to changes in the pressure of air and fuel in the intake of the engine,- a pressure operated mechanism responsive to variations in said pressure for causing said resistance to so respond, said bridge having in another and opposed branch a resistance responsive to changes in the temperature of the exhaust gases of said engine, a null point electrical meter connected across said. bridge for registering unbalance of said bridge when the same is'afiected by changes I in either of said variable resistances.

2. A device for indicating the adjustment of air to fuel ratios in internal combustion engines comprising in combination a source of electrical connected in parallel with one another and in series with said source, one of said resistan'cesbeing responsive to changes in the pressure of air and fuel in the intake of said engine and the other being responsive to changes in the temperature of the exhaust gases of said engine, and a null point electrical meter connected across said bridge for registering unbalance of said bridge when the same is affected by changes in either of said variable resistances.

3. A combustion guide for internal combustion engines comprising a source of electric current, a Wheatstone bridge arrangement connected with said source, a meter connected across the Wheatstone bridge, a closed chamber communicating with the intake manifold of the engine, a pressure responsive device in said chamber, and a second closed chamber communicating with the gases of combustion constituting the exhaust from said engine, opposed arms of said bridge being separately disposed within each of said current, a Wheatstone bridge connected thereto, said bridge having a pair of variable resistances stone bridge, a closed chamber communicating with the intake manifold-oi the engine, a pressure responsive device in said chamber, and a second closed chamber communicating with the gases'of combustion constituting the exhaust from said engine. opposed arms of said bridge being separately disposed within each of said chambers, said arms being in parallel with one another and in series with the said source of current. and being adapted and arranged to indicate on the meter variations in the electrical current'in said arms in response to conditions within said chambers, said meter being operated by variations in the resistances of the Wheatstone bridge whereby the difierential between the intake manifold pressure and the exhaust manifold temperature is indicatedon said meter.

5. A combustion guide for internal combustion engines comprising a Wheatstone bridge includ-.

.ing a source of current therefor, a meter connected across the Wheatstone bridge, a closed chamber communicating with-the intake manifold of the engine, a pressure-responsive device in said chamber, one arm of said bridge being disposed in said chamber and another arm being disposed in thermal relation with the exhaust bridge whereby the differential between the in-- take manifold pressure and the exhaust manifold temperature is indicated on said meter.

6. Means for correlated registrationof the quantity and intensity combustion factors in in,- ternal combustion engines which comprises a resistance. element in the engine exhaust and connected in one branch of a Wheatstone bridge, a variable resistance connected in the other branch tion of the product of said quantity and intensity factors.

' CLARKE C. MINTER. 

